The long-term goal of the proposed research is to develop a novel quantitative flow through cell counting method for enumeration of CD4+ T lymphocytes in human blood. CD4 cell counts are needed to stage and monitor HIV-infected patients. In adults, the absolute number of CD4+ T cells per microlitre of blood has critical prognostic and therapeutic implications, and is used for both HIV staging and treatment decisions. The gold standard technique currently used for quantitation of CD4 cells is flow cytometry. However, flow cytometers, including smaller single-purpose CD4 cell counting devices developed recently are expensive and inaccessible in resource-limited countries. In addition, they require trained laboratory technicians to handle and process samples and to analyze and report data. Because flow cytometry-based CD4 cell counting is beyond the reach of so many HIV-infected people, the World Health Organization (WHO) has urged the development of simple point-of-care CD4 cell count assays designed for resource-limited settings. It is in this context that the less expensive, rapid and easy-to-perform method, which is free from the limitations of the conventional assays, for determination of CD4+ T cells is proposed to be developed. The proposed method, flow-through cell counting assay (FTCA) is based on the use of the special membrane and a proprietary flow-through cassette to measure cell count status. The test can potentially be completed in less than 15 minutes and can be performed by laboratory technicians with low levels of training. The specific aims of Phase II are: 1) To design and assemble a pre-production -prototype of the FTCA cassette and adapt a blood glucose meter for the FTCA signal measurements; 2) To explore all variables affecting the performance of the FTCA CD4+ T-cell counting method and optimize the performance of the assay; 3) To prepare lyophilized immuno-reagents for performing FTCA and examine the stability of all FTCA reagents; 4) To validate the FTCA by examining a statistically significant number of clinical samples. Advantages of the proposed technology such as the speed, convenience, and low cost of the analysis substantiates our belief that the FTCA will be widely used in clinical practice.